A known conventional switching power source apparatus is shown in FIG. 1. Operation of the conventional switching power source apparatus 100 as shown in FIG. 1 will be explained.
An AC power source 1 is applied to the power source apparatus, and a rectifier 3 full-wave-rectifies a sinusoidal voltage supplied from the AC power source 1 and supplies the full-wave-rectified waveform to this chopper circuit
Initially, an end of a secondary coil 5b provided for a step-up reactor 5 is connected to GND, and the other end thereof is connected to a inverting (−) input terminal of a comparator 13 through a resistor R13. At the same time, a non-inverting (+) input terminal of the comparator 13 receives a reference voltage Vref2. The comparator 13 compares the input voltages with each other, and if the reference voltage Verf2 inputted to the non-inverting input terminal is greater than a detect voltage value of the secondary coil 5b inputted to the inverting input terminal, outputs a high-level set signal to a flip-flop 15.
The flip-flop 15 is set in response to the set signal from the comparator 13, and a Q output terminal outputs a high-level drive signal to turn on a switching element Q1. When the switching element Q1 is turned on, a switching current flows through a primary coil 5a of the step-up reactor 5, the drain-source of the switching element Q1, and a current detecting resistor R9 to GND, to thereby accumulate energy in the step-up reactor 5.
At this time, the switching current flowing through the switching element Q1 is converted into a voltage Vs by the current detecting resistor R9 being arranged between the source of the switching element Q1 and GND. The voltage Vs is inputted to a non-inverting (+) input terminal of a comparator 11, which compares it with a current target value Vm output from a multiplier 9.
When the voltage-converted value Vs of the switching current reaches the current target value Vm, the comparator 11 outputs a high-level reset signal through an OR circuit 23 to the flip-fop 15. The flip-flop 15 is reset in response to the reset signal being received from the comparator 11 through the OR circuit 23, to change the drive signal output from the Q output terminal from the high level to a low level to turn off the switching element Q1.
When the switching element Q1 is turned off, the energy accumulated in the step-up reactor 5 is combined with a voltage supplied from the rectifier 3, to charge an output capacitor C1 through a rectifying diode D5. As a result, the output capacitor C1 receives a DC voltage that has been stepped up higher than a peak value of the full-wave-rectified waveform supplied from the rectifier 3.
The voltage applied to the capacitor C1 is divided by resistors R5 and R7 and is inputted to an error amplifier 7, which compares it with a reference voltage Vref1 and supplies an error voltage Ver to the multiplier 9.
The full-wave-rectified waveform from the rectifier 3 is divided by resistors R1 and R3 and is inputted to the multiplier 9, which multiplies it by the error voltage and supplies the current target value Vm for a switching current to the inverting (−) input terminal of the comparator 11.
Next, when the discharge of the energy accumulated in the step-up reactor 5 ends, a voltage induced at the secondary coil 5b inverts. This voltage is compared with the reference voltage Vref2 in the comparator 13, which outputs a high-level set signal to the flip-flop 15. In response to the set signal from the comparator 13, the flip-flop 15 is set to again supply a drive signal to turn on the switching element Q1.
Thereafter, such operation is repeated to maintain an output voltage of the output capacitor C1 at a constant value. At the same time, a current of the AC power source 1 becomes a sinusoidal current waveform based on the voltage of the AC power source 1.
Next, basic operation of an overvoltage protection circuit 16 arranged in the conventional switching power source apparatus 100 will be explained.
In the overvoltage protection circuit 16, a voltage at a point B that divides the output voltage Vo by resistors R15 and R17 is input to a cathode of a Zener diode ZD1.
If the voltage at the point B is lower than a Zener voltage of the Zener diode ZD1, the Zener diode has a high impedance not to pass a current to a resistor R19, and therefore, a voltage at a non-inverting (+) input terminal of a comparator 17 is lower than a reference voltage Vref3. Accordingly, the comparator 17 provides a low-level output, and a Q output terminal of a flip-flop 21 provides a low-level output As a result, a high-level reset signal being periodically outputted from the comparator 11 is supplied to the flip-flop 15 through the OR circuit 23. Consequently, the output voltage of the output capacitor C1 is maintained at a constant level as mentioned above.
On the other hand, if the voltage at the point B is higher than the Zener voltage of the Zener diode ZD1, a current is passed to the resistor R19, and therefore, a voltage at the “+” input terminal of the comparator 17 becomes higher than the reference voltage Vref3. Then, the comparator 17 outputs a high-level signal to set the flip-flop 21. Then, the Q output terminal of the flip-flop 21 outputs a high-level signal. The high-level signal output from the Q output terminal of the flip-flop 21 is input through an OR circuit 19 to a set terminal S of the flip-flop 21, and therefore, the Q output terminal of the flip-flop 21 outputs a high-level reset signal through the OR circuit 23 to a reset terminal R of the flip-flop 15. The reset signal is held at the reset terminal R. As a result, the ON operation of the switching element Q1 stops, and the output voltage of the output capacitor C1 drops to 0 V and is kept at there.
Next, an input voltage varying test, a limit input voltage test, and a ring wave test conducted on the conventional switching power source apparatus 100 will be explained.
(1) Input Voltage Varying Test
The input voltage varying test supplies the AC power source 1 in the range of AC 90 V to AC 264 V, and in a case where the output voltage Vo is, for example, DC 380 V, it is determined that operation of the apparatus is normal.
(2) Limit Input Voltage Test
The limit input voltage test supplies the AC power source 1 of AC 400 V at the maximum, and in a case where the output voltage Vo is a DC voltage greater than, for example, an effective value of the input voltage, it is determined that operation of the apparatus is normal.
(3) Ring Wave Test
The ring wave test supplies an AC voltage (480√2V) formed by superimposing a pulse signal of 240√2V on an AC power source of, for example, AC 240 V, and in a case where the output voltage Vo is a DC voltage of, for example, 380 V, it is determined that operation of the apparatus is normal.